What do you mean by “jumpy” in regards to the camera’s motion? That might help with finding a solution!
In-case this might be the fix: The mouse won’t move the camera if the cursor exits the bounds of the window. When the mouse exits and re-enters the window area, it will cause a jump. If you want the mouse to stay in the window (and thus always move the camera without issue) you can add this to the ready() function of your script:
Input.mouse_mode = Input.MOUSE_MODE_CAPTURED
Note that this will hide the mouse as well. You will need to alt-tab or force close the game window (via Alt-F4) in order to regain control of the mouse.